Sol-gel method for preparing an alkaline earth metal borate

ABSTRACT

The present invention concerns a method for preparing earth alkaline metal borates. 
     The method comprises the steps of forming a mixture of an earth-alkaline metal alkoxide and a boron alkoxide, of hydrolysing this mixture to form a metal-borate precursor, and then coating the precursor onto a substrate and thermally treating it to form an inorganic film on the substrate. 
     The method is especially useful to prepare beta barium borate layers which can serve in non-linear optical devices to produce second harmonics generation.

The present invention concerns a method for preparing alkaline earthmetal borates and articles comprising such borates.

Beta-barium borate (β-BBO) is one of the materials having non-linearoptical properties, and more particularly having the capacity in acertain range of wavelengths, to produce a second harmonic generation(frequency 2x) when it is submitted to an incident radiation offrequency x. Unlike other materials with usual sources of incidentradiation, this second harmonic generation (SHG) is of sufficientamplitude to be observed and manipulated.

Another advantage of β-BBO is that its optical damage threshold is high,i.e. 3-4 times the threshold of potassium dihydrogenophosphate, which isused as a reference material in non-linear optics. This means that β-BBOcan be used with short pulse, high power lasers as sources of incidentradiations without being damaged. Also, β-BBO can be used for non-linearoptics within a range of temperatures which is broader than for anyother material.

However, to be actually usable as a non-linear integrated opticaldevice, a material has also to be obtainable in crystalline thin filmform, on appropriate substrates.

Substances such as BBO are usually prepared by solid state reaction andsintering of metal oxide and metal carbonate mixtures. However, thetreatment of these physical mixtures requires prolonged grinding/heatingcycles. These methods are complex, expansive, provide non-uniform filmswith uncontrolled stoichiometry and do not permit easily to introducedopants in a quantitatively controlled manner. Some of these problemsmay be partially overcome by tedious and time consuming trial and errorcombinations of the chemical elements in question, in order to achievematerials and films with the desired composition ratios. Besides,certain alkoxides have an alkaline pH, and hydrolysis at alkaline pHproduces powders; a stable solution is usually obtainable with theaddition of acids. However, the undesirable consequence of the additionof acids is an inferior crystallinity and the increase ofcrystallization temperatures when the film is subjected to thermaltreatments.

Because of these difficulties, there has been a strong interest in thepreparation of oxide ceramics by chemical methods in order to achievethe desired stoichiometric composition and atomic level homogeneity, andto reduce processing time and temperatures. One of such methods issol-gel method, which consists in a controlled hydrolysis of precursorswhich are generally alkoxides. These alkoxides are hydrolyzed andcondensed to form an inorganic metal-oxygen-metal network. Furtherthermal treatments result in the formation of the desired crystallinephase. Sol-gel method has been described in references such as U.S. Pat.Nos. 4,789,563, 4,652,467, 4,849,252 and EP's 130,801, 232,941. Varioushydrolysis methods are reported in the technical literature.

Chem. Lett. 11 (1979), 1341-1344, discloses the preparation of strontiumborate by pouring a mixture of boron and strontium alkoxide into aqueousammonia solution to form the precursor, which is decomposed by heatingto form the borate.

Z. anorg. Allg. Chem. 460 (1980), pages 228-234, discloses thepreparation of borates by adding alkaline earth metal to a watercontaining mixture of boron alkoxide in alcohol followed by boiling toobtain borate precursors.

JP-A-63293178 discloses the preparation of complex metal and boroncontaining compounds by reacting boron alkoxide, metal alkoxide, alcoholamines and carboxylic acid in an organic solvent. Alkaline earth metalsand pH conditions are not disclosed. However, a requirement for thesol-gel method is that alkoxides must be hydrolyzed in a controlledmanner. Boron alkoxides are very water-sensitive, i.e. they have atendancy to hydrolyse too rapidly and to produce powders; thus, sol-gelmethod was thought not to be a good candidate for preparing β-BBO, orrelated borates.

The present invention eliminates the above difficulties and provides away to use the sol-gel method for preparing alkaline earth metal boratessuch as β-BBO with precise atomic ratios on various substrates andwithout requiring expensive and complex deposition systems or thepreliminary preparation of powders of the desired material.

The method according to invention allows to form a borate of one or moreearth alkaline metal and comprises the steps of:

(a) mixing a earth alkaline metal alkoxide with a boron alkoxide in thepresence of a volatile solvent,

(b) hydrolyzing this mixture at a pH between 5 and 9, and at roomtemperature, to form an earth alkaline metal borate precursor,

(c) thermally treating the earth alkaline metal borate precursor to forman earth alkaline metal borate.

According to a preferred embodiment, the method of the invention allowsto form a film of a borate of one or more earth alkaline metals on asubstrate and comprises the steps of:

(a) mixing a earth alkaline metal alkoxide with a boron alkoxide in thepresence of a volatile solvent,

(b) hydrolyzing this mixture at a pH between 5 and 9, and at roomtemperature, by adding slowly a mixture of water and volatile solvent,

(c) coating the resulting solution on a substrate, and

(d) thermally treating the coated solution so that the solvent and waterare removed and a inorganic thin film of metal borate is formed.

The term "metal" refers to any alkaline earth metal, i.e. a metal ofgroup 2 of the Periodic Table such as, calcium, strontium and barium.

According to a specific and preferred embodiment, the method of theinvention is especially suitable to prepare beta BBO, and comprises thefollowing steps:

(a) preparation of a mixture of barium alkoxide with a boron alkoxide inthe presence of a volatile solvent and at pH 6-8,

(b) hydrolysis of this mixture to form a β-BBO precursor,

(c) coating of the precursor obtained in step (b) on a substrate; and

(d) thermal treatment of the precursor, in order to form β-BBO.

While the Applicants do not intend to be bound by theoriticalconsiderations, they believe than an important feature of the methodaccording to the invention is the choice of alkoxides which can undergoa controlled hydrolysis at a substantially neutral pH so as to produceby in-situ polymerization a stable, water-stable and non volatilpolymeric network consisting of metal-oxygen-metal bonds, which can bethen coated in thin film and thermally treated.

The solvents can be chosen from a wide range of liquids which arevolatile at room temperature. The solvent should allow solubilization ofthe alkoxides used. The choice of the solvent (or solvents) and theamount of solvent(s) will depend on requirements such as viscosity andsurface tension of the solution obtained after the hydrolysis. Viscosityand surface tension are parameters that determine the wettability andcoatability of the solution on a substrate. Preferred solvents arealcohols and more preferred are lower alkanols, still more preferablyC1-C10 alkanols, such as methanol, ethanol, propanols, butanols or loweralkoxy alkanols such as lower alkoxy ethanols, etc.

If desired, the surface tension can be reduced by the addition of asurfactant. Various nonionic or ionic surfactants can be used for thispurpose, such as those sold under the trademark Triton by Rohm & Haas,e.g. Triton Xn (n=15, 35, 45, 100, etc).

Various boron alkoxides can be used to form the metal borate precursor.Preferred ones are those derived from lower aliphatic alcohols, i.e.those having from 1 to 8 carbon atoms such as methoxide, ethoxide,propoxide, butoxide, amyloxide etc.

An important feature of this invention is that the alkoxides chosen canbe hydrolyzed in a controlled manner by the sol-gel process, at roomtemperature and at a pH between 5 and 9, preferably between 6 and 8 andmore preferably close to the neutrality, in order both to avoidexcessive alkalinity which causes the hydrolysis to produce powders, andwithout the addition an acids which leads to higher crystallizationtemperature and poor crystallinity. The hydrolysis is preferably carriedout with a mixture of water and solvents, such as one or more of theaforementioned solvents. Molar ratio of water to solvent can vary withina wide range and can be preferably comprised between 0:1 and 10:1, morepreferably between 1:1 and 5:1.

As previously noted, the metal alkoxide compounds and the boron alkoxideare used in the proportions desired in the final thin crystalline film.The compounds can be incorporated in the solvent in any convenientconcentration at ambient temperature. Generally, a concentration ischosen which provides the desired film thickness for the processsequence.

Where the geometry of the substrate permits, uniformity and thickness ofthe metal-ligand coating can be controlled by spinning the substratearound an axis normal to the surface of the substrate. A significantadvantage of spin coating is that the thickness of the coating at theend of spinning is determined by the contact angle and viscosity of thecoating composition and the rate and time of spinning, all of which canbe precisely controlled. Differences in the amount of the coatingsolution applied to the substrate are not reflected in the thickness ofthe final coating. Centrifugal forces generated by spinning cause excessmaterial to be rejected peripherally from the article.

According to the method of the invention, after it has been hydrolyzed,the solution containing barium and boron at the desired ratios used asthe precursor, is coated onto the substrate by any known coating method,such as spin coating, dip coating, spraying, doctor blading, etc.Thicker films can be obtained by repeating the coating steps. Thethickness of the films depend on factors such as the viscosity of thesolution prior to coating.

The resulting film is then subjected to a thermal treatment in one ormore sequences, to remove solvents, water, and residual organicsreleased from the alkoxides. Before heating at higher temperatures forremoval of residual organics, a heating step under water steam at amoderate temperature can be desirable (for instance between 200° and300° C.) in order to have the hydrolysis completed. An uncompletehydrolysis may cause difficulties for the complete removal of residualorganics at higher temperatures. The heating steps finally result in aninorganic film, which upon continued thermal treatment, is modified toproduce the crystalline desired phase of metal borate. In the case ofbeta BBO films, after hydrolysis and condensation, the expectedbarium-boron-oxygen coordination is achieved in liquid phase until thecrystalline β-BBO phase is obtained without boron loss during theheating treatments. The deposition process can be repeated one orseveral times with the removal of water and solvents after eachdeposition step.

Processing temperatures employed in forming the inorganic film and insubsequently converting it to a crystalline film can vary significantly,depending upon the specific film composition. In a preferred embodiment,crystallization is achieved at temperatures below 1000° C. and down toabout 500° C. Optimal temperatures for crystallization may vary,depending on the ratios of the components of the film. Crystallizationof β-BBO can be achieved at temperatures between 600° C. and 920° C.

To heat the film and the substrate, various methods can be used. Uniformheating can be accomplished employing any conventional oven. In someinstances, however, either to protect the substrate from rising to thepeak temperatures encountered by the film or simply to avoid theinvestment in an oven, it is contemplated that the film will beselectively heated. This can be accomplished by employing a radiant heatsource such as a lamp, e.g. a quartz lamp. Lamps of this type arecommercially available for achieving rapid thermal annealing of variousconventional layers and can be readily applied to the practice of theinvention. These lamps rapidly transmit high levels of electromagneticenergy to the film, allowing it to be brought to its crystallizationtemperature without placing the substrate in an oven.

After thermal treatment the layer thickness can range from about 0.05 μmto about 0.5 μm and preferably from about 0.1 μm to about 0.3 μm.Thickness are measured by methods such as ellipsometry.

Preferred substrates for the film are those which are inert or minimallyinteractive with the metal-borate film deposited thereon. It isgenerally preferred to select substrates from among materials whichexhibit relatively limited interaction with the metal borate film duringits formation. Description of useful substrates is provided in EuropeanPatent 334,093 or in U.S. Pat. No. 5,017,551. Also, MgO and sapphiremonocrystals can be used as substrates.

To avoid the interaction between the film and the substrate, it is alsopossible to insert a barrier layer, e.g. layer of boron silicate betweenthe substrate and the film.

The following examples illustrate the invention.

EXAMPLE 1 Preparation of Barium Alkoxide

24 g of metal barium in the form of pellets were slowly added to 250 mlof 2-methoxyethanol (about 0.7 molar). The exothermic reaction producesa hydrogen evolution. As the barium pellets dissolve quite slowly, thesolution darkens up to exhibit a dark colouring. At the end of thereaction, the solution is cleared by filtration. The solution which willbe used as such in the following step, contains 0.77 molar bariumalkoxide.

EXAMPLE 2 Preparation of B-barium Borate

10 ml of the barium 2-methoxyethoxide solution in 2-methoxyethanolobtained in Example 1 (0.77×10⁻² mole of barium) were mixed with 3.54 gof boron tributoxide (1.54×10⁻² mole), under an inert atmosphere(nitrogen).

A mixture of water (1.11 g; 6.16×10⁻² mole) and solvent(2-methoxyethanol, 8 ml) was slowly added; the water: solvent ratio is1:1.

The resulting sol was heated to reflux at 50° C. during 24 hours, andthen is coated as on an alumina substrate.

The film was then subjected to three successive thermal treatments:

A first heating during 5 minutes at 150° C. in order to remove thesolvent and water;

A second heating during 24 hours at 250° C. in the presence of water (inan oven).

A third heating during 15 minutes at 820° C. in order to performcrystallization. The film crystallization was verified by X-raydiffraction, and electron scanning microscopy.

EXAMPLE 3 Preparation of B-barium Borate

The procedure of Example 2 is repeated, except that 15 ml (1.155×10⁻²mole) of barium 2-methoxyethoxide were used and mixed with 1.28 g(0.77×10⁻² mole) of trimethoxyboxine (B₃ O₃ (OCH₃)₃.

The hydrolysis was carried out with a mixture of 0.83 of water and 12 mlof 2-methoxyethanol. A sol treated as in example 2 was obtained in orderto form, after heating, a β-barium borate film.

We claim:
 1. A method for preparing a borate of one or more alkalineearth metals, comprising the steps of(a) forming a mixture of a boronalkoxide and one or more alkaline earth metal alkoxides in the presenceof a volatile solvent, (b) hydrolyzing said mixture at a pH between 5and 8 by adding slowly a mixture of volatile solvent and water, to forma alkaline earth metal borate precursor, (c) coating the resultingcomposition of step (b) on a substrate, and (d) heating the coatedcomposition of step (c) so that the solvents and water are removed andan inorganic thin film of alkaline earth metal borate is obtained. 2.The method of claim 1, wherein the mixture formed in step (a) alsocontains a soluble solvent which is volatile at room temperature.
 3. Themethod of claim 1, wherein the mixture formed in step (a) is hydrolyzedin the presence of a room-temperature volatile solvent.
 4. The method ofclaim 1 wherein the mixture formed in step (a) contains a volatilesolvent which is an alcohol and said mixture is hydrolyzed by addingslowly a mixture of water and said alcohol.
 5. The method of claim 1,wherein the alkaline earth metal is barium and the resulting borate isbeat barium borate.
 6. The method of claim 5, wherein the boron alkoxideis selected from the group consisting of methoxide, ethoxide, propoxide,butoxide and trimethoxyboxine.
 7. The method of claim 5, wherein thebarium alkoxide is selected from the group consisting of2-methoxyethoxide and methoxide.
 8. The method of claim 1, wherein theheating step comprises at least one step for removal of water andsolvents, a step for the completion of hydrolysis, a step for theremoval of residual organics and a step for the crystallization of themetal borate.
 9. The method of claim 8, wherein the water and solventremoval step is repeated after each deposition of the materials on thesubstrate.